Dispersion compensation device and dispersion compensation method

ABSTRACT

A dispersion compensating fiber whose chromatic dispersion is positive and a negative dispersion compensating fiber whose chromatic dispersion is negative are prepared, and division-multiplexed optical signals, after being guided to either dispersion compensating fiber to once shift the whole wavelength band to positivity or negativity, are subjected to fine adjustment with a dispersion compensating fiber of a reverse sign.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to Japanese patent application Ser. No.2004-365893, filed on Dec. 17, 2004, entitled “Variable DispersionCompensation Device, Optical Transmission System Using It and Method ofSetting Dispersion Compensation Quantity” the content of which areincorporated herein by reference.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationSer. No. 2004-375729, filed on Dec. 27,2004, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dispersion compensation device and adispersion compensation method, and more particularly to a dispersioncompensation device and a dispersion compensation method permittingdispersion compensation for dispersion-shifted fibers.

2. Description of Related Art

When designing a wavelength division multiplexing (WDM) system whichpasses wavelength division-multiplexed signals over a single mode fiber(SMF) transmission line, it is necessary to apply dispersioncompensating fibers (DCFs) to compensate for chromatic dispersion andchromatic dispersion slope. Since the chromatic dispersion of an SMF isabout +20 ps/nm/km in a 1.55 μm communication band as shown in FIG. 1,the DCFs are selected out of fibers having negative chromatic dispersionof, for instance, −80 to −100 ps/nm/km.

Apart from them, there are also deployed dispersion-shifted fibers(DSFs) formed by shifting the zero-dispersion wavelength, which is 1.3μm (micrometers) for SMFs, to 1.55 μm, which belongs to thecommunication band. Generally,DSFswere assumed to require no dispersioncompensation, but their dispersion compensation has also come to beincreasingly called for on account of such recent developments as theincrease in transmission speed, extension of transmission distance,further densification of dense WDM (DWDM) and realization of transparentnetworks with no or little regeneration.

In determining the appropriate dispersion compensation quantity, thedispersion value on the optical fiber transmission line is actuallymeasured or predicted, and DCFs of appropriate lengths are mounted onthe transmission device. However, since this necessitates keeping manydifferent kinds of DCFs in stock, there also is a keen requirement forvariable dispersion compensation devices.

U.S. Pat. No. 5,930,045 (Patent Reference 1) describes a variabledispersion compensation device using a virtual image phase array(VIPA).U.S. Pat. No. 5,608,562 (Patent Reference 2) describes a variabledispersion compensation device using an optical circulator, pluraloptical switches, plural DCFs and a mirror.

The C-band used for communication in optical transmission ranges from1530 to 1565 nm. The zero-dispersion wavelength of DSFs is 1.55 μm. Thismeans that, when WDM transmission is to be done by using DSFs, lights of1530 nm are subject to negative chromatic dispersion and lights of 1565nm are subject to positive chromatic dispersion. DSFs are also subjectto inevitable fluctuations in the manufacturing process, and thereforetheir zero-dispersion wavelength may differ from 1550 nm.

The VIPA described in Patent Reference 1 represents a technology thatcan provide both positive chromatic dispersion and negative chromaticdispersion, but it requires assembly of a complex optical system andaccordingly is expensive.

The invention described in Patent Reference 2 is mainly composed ofpassive optical components. However, the optical switches used involveelements to make the configuration complex, including a large number ofcontacts, and accordingly may fail to maintain its reliability level.Since main signals pass every optical switch whether they travel pastthe DCFs or not, if the optical switches and the DCFs are unitized, theunits cannot be detached or diverted to any other purpose. Thus, if thedispersion quantity is to be altered by remote control when any DCF isto be added, every optical switch that maybe addedwill have to beinstalled in advance, or if any optical switch is to be added orremoved, main signal transmission should be suspended during the addingor removing work. Furthermore, Patent Reference 2 makes no mention ofdispersion compensation on a transmission line using DSFs. Moreover, ithas to be equipped with several kinds of DCFs.

In general terminology, DSF means a fiber whose chromatic dispersionfalls off to zero in the vicinity of 1.55 μm, but similar fibers, whichare non-zero dispersion compensating fibers (NZ-DSFs) having chromaticdispersion of a few ps/nm/km in the vicinity of 1.55 μm, are also used.These fibers may also pose the same problem. NZ-DSFs are also DSFs in abroader sense of the term.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a dispersioncompensation device and a dispersion compensation method permittingcollective dispersion compensation of wavelength division-multiplexedoptical signals having propagated over a transmission line involvingsuch DSFs and other elements and permitting ready addition or removal ofelements.

According to the invention, one end of a first fiber having a positivechromatic dispersion characteristic and one end of a second fiber havinga negative chromatic dispersion characteristic are connected to eachother, optical signals are inputted from another end of the first fiber,optical signals are outputted from another end of the second fiber, andchromatic dispersion of the optical signals is thereby compensated for.

Alternatively, optical signals may be inputted from the other end of thesecond fiber and outputted from the other end of the first fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates the characteristics of various optical fibersrelative to the wavelength;

FIGS. 2 illustrate networks to which the invention is to be applied;

FIGS. 3 illustrate inter-station transmission systems to which theinvention is to be applied;

FIGS. 4 illustrate the effects of dispersion compensation;

FIG. 5 is a block diagram of avariable dispersion compensation device,which is a preferred embodiment of the invention;

FIGS. 6 illustrate the dispersion quantity and fiber length of adispersion compensation unit in the preferred embodiment of theinvention;

FIG. 7 is a hardware block diagram of the dispersion compensationdevice, which is another preferred embodiment of the invention; and

FIG. 8 shows an external view of the dispersion compensation device,which is this other preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedbelow with reference to the accompanying drawings. Of these drawings,FIGS. 2 illustrate networks to which the invention is to be applied;FIGS. 3 illustrate inter-station transmission systems to which theinvention is to be applied; FIGS. 4 illustrate the effects of dispersioncompensation; FIG. 5 is a block diagram of a variable dispersioncompensation device, which is a preferred embodiment of the invention;FIGS. 6 illustrate the dispersion quantity and fiber length of adispersion compensation unit in the preferred embodiment of theinvention; FIG. 7 is a hardware block diagram of the dispersioncompensation device, which is another preferred embodiment of theinvention; and FIG. 8 shows an external view of the dispersioncompensation device, which is this other preferred embodiment of theinvention.

The topology of the network will be described with reference to FIGS. 2.FIG. 2A shows a point-to-point (P-to-P) network linking two terminalstations 10A and l0B by a DSF. FIG. 2B shows a ring network, a typicalexample of which is SDH. In a ring network, each of a station 10A, astation 10B, a station 10C and a station 10D is linked to immediatelyadjoining stations by DSFs. Since the network constitutes a ring, evenif a fiber runs into a fault in one position, protection can be providedby transmission in the reverse direction. Moreover, the networkoperation is relatively simple. In a meshed network shown in FIG. 2C,the station 10A, the station 10B, the station 10C, the station 10D andanother station (not shown) are connected by DSFs in a mesh shape. Ameshed network, though allowing the greatest degree of designing freedomincluding compatibility with redesigning to match various conditions, isdifficult to operate and manage.

A variable dispersion compensation device, which is the preferredembodiment of the invention, is arranged in each station 10 andcompensates for any chromatic dispersion arising on an optical fibertransmission line. If any fault occurs on the transmission line directlylinking the station 10A and the station 10D as illustrated in FIG. 2C,the transmission line passing the station 10B, for instance, is longerin distance and involves greater chromatic dispersion than the directlylinking transmission line in the transparent network referred to above.Even in such a case, a variable dispersion compensation device couldrespond promptly. The effect of the invention would be particularlysignificant at a transmission speed of 10 Gbps, 10.7 Gbps or 40 Gbpswhere the need for dispersion compensation is keener, but the inventioncan be expected to prove effective at any other transmission speed ifthe total dispersion quantity, which is determined by the transmissiondistance and the type of fiber, surpasses the dispersion endurance ofthe receiver.

Next will be described inter-station transmission systems with referenceto FIGS. 3. Though FIGS. 3 depict the P-to-P type shown in FIG. 2A,transmission in this system is equivalent to transmission between anytwo stations in a ring network or a meshed network.

Referring to FIG. 3A, the terminal station on the transmitting sideincludes n optical transmitters 20, a wavelength multiplexer 30 forwavelength-multiplexing optical signals differing in wavelength (λ1, λ2,. . . , λn) from the optical transmitters 20, and optical amplifiers 40.The terminal station on the receiving side, on the other hand, includesoptical amplifiers 40, a variable dispersion compensation device 100, awavelength demultiplexer 50 for wavelength-demultiplexingwavelength-multiplexed optical signals, and n optical receivers 60. Theterminal station on.the transmitting side and the terminal station onthe receiving side are connected by DSFs 70. About midway on each leg ofthe DSFs, there is a through node, where an optical amplifier 40 isinstalled.

FIGS. 3 illustrate different arrangements of the variable dispersioncompensation device within the transmission system. In the configurationof FIG. 3A, it is arranged in the terminal station on the receivingside; in that of FIG. 3B, in the through node; and in that of FIG. 3C,in the terminal station on the transmitting side. Especially theconfiguration shown in FIG. 3C in which the dispersion compensator islocated in the terminal station on the transmitting side, dispersioncompensation on the transmission line on which signals are to betransmitted is accomplished in advance. The device may be arranged inany two stations or every station in a disperse way. The positionalrelationship between the optical amplifier and the dispersioncompensator is not limited to this. They may be installed at a prior orposterior stage or even built in.

Chromatic dispersion and dispersion compensation on the transmissionline will be described with reference to FIGS. 4. This calculation ismade in terms of an SMF. FIG. 4A shows a profile of a transmittedsignal, whose waveform is normalized to intensity 1 in the Gaussiandistribution. The full-width half-maximum of this waveform is about 40ps. FIG. 4B shows a waveform after transmission over an SMF of 17ps/nm/km in chromatic dispersion over a distance of 80 km. The intensityis 0.72, and the full-width half-maximum is about b 80 ps. What resultsfrom dispersion compensation at −1360 ps/nm (=−17 ×80 ps/nm) on thewaveform of FIG. 4B is the waveform of FIG. 4C.

The variable dispersion compensation device, which is the preferredembodiment of the invention, will now be described with reference toFIG. 5. Referring to FIG. 5, wavelength division-multiplexed mainsignals are inputted through a port 111 of an optical circulator 110,and outputted from a port 112 of the optical circulator 110. To the port112 of the optical circulator 110, n dispersion compensation units, eachformed of a dispersion compensating fiber 120, a 1 ×2 optical switch 130and a mirror 140, are connected in series. Each 1 ×2 optical switch 130here has one input; the mirror 140 is connected to one of its outputs,and the input to the dispersion compensation unit of the next stage isconnected to the other output. The dispersion compensation unit of then-th stage has no 1 ×2 optical switch, but a mirror is directlyconnected to its dispersion compensating fiber.

Where this configuration is adopted, optical signals are turned back bya mirror at the dispersion compensation unit stage where the mirror hasbeen selected under the control of the 1 ×2 optical switch, and inputtedto the port 112 of the optical circulator 110. The optical signalsinputted to the port 112 of the optical circulator 110 are outputtedfrom a port 113 of the optical circulator 110.

Thus, the optical signals, while they are inputted to the port 111 ofthe optical circulator 110 and outputted from the port 113, twice passesthe dispersion compensating fiber of each stage until the stage wherethe mirror has been selected and undergoes dispersion compensation.Incidentally, where the transmission line is formed of an SMF, thedispersion compensating fiber is a fiber of negative chromaticdispersion. Therefore, simple reference to dispersion compensatingfibers would mean fibers of negative chromatic dispersion in thecommunication wavelength band. Yet the term in the context of thisembodiment is not limited to them, but also covers fibers of positivechromatic dispersion in the communication wavelength band.

As the use of commercially available latch type optical switches usingpermanent magnets or electromagnets as the optical switches here couldenable connection to be maintained even in the absence of power supply,power consumption can be saved.

The dispersion quantity and fiber length of each dispersion compensationunit will be described with reference to FIGS. 6. Referring to FIGS. 6,a row 61 for the dispersion quantity of each unit states the dispersionquantity of the dispersion compensating fiber 120 of each dispersioncompensation unit. A row 62 for the reference numbers of the opticalswitches to be switched to the mirror states the 1 ×2 switch No. of eachdispersion compensation unit. A row 63 for the total dispersioncompensation quantities in main signal output units states the totaldispersion compensation quantities in the optical signals outputted fromthe ports 113 when the signals are turned back at each dispersioncompensation unit. A row 64 for fiber lengths states the lengths of thedispersion compensating fibers. A row 65 for the total fiber lengthsstates the total fiber length of each dispersion compensation unit whenthe signals areturnedbackat theparticulardispersioncompensation unit.The total fiber length will be described with reference to FIG. 7. Thecontents of FIGS. 6 are recorded in a nonvolatile memory illustrated inFIG. 7.

FIG. 6A shows a case in which the number of stages of the dispersioncompensation units is 5, with a compensation quantity of 500 ps/nm beingset for the first dispersion compensation unit 1, and the compensationquantity for each of the dispersion compensation unit 2 through thedispersion compensation unit 5 being reversed in sign, −250 ps/nm inparticular. As a result, the range of compensation by the variabledispersion compensation device is from 1000 ps/nm to −1000 ps/nm at 500ps/mn graduations. According to the invention, achieving the variablerange and the graduations of variation stated above requires only twodispersion compensation units, one of 500 ps/nm and the other of −250ps/nm, and the configuration is thereby simplified.

The reason for the plus/minus difference in dispersion compensationquantity between the first dispersion compensation unit and thefollowing dispersion compensation units will be described below.Referring back to FIG. 1, a DSF differs in the required direction ofdispersion compensation, positive or negative, with the wavelength inthe wavelength band used for communication. In FIG. 6A, in the wholewavelength band, the compensation is once shifted in the positivedirection of the y axis, followed by fine adjustment in the negativedirection of the y axis. The first dispersion compensation unit ispassed again at the final stage after reflection by a mirror. However,the total of the two passages can be regarded as the shift in thepositive direction irrespective of the positions in the sequence ofdispersion compensation.

Now referring to FIG. 6B, thenumberof dispersioncompensation units isfive, with a compensation quantity of −500 ps/nm is set to the firstdispersion compensation unit 1 and one of 250 ps/nm to each ofdispersion compensation units 2 through 5, reversed to the positive. Asa result, the compensation range by the whole variable dispersioncompensation device is set between −1000 ps/nm and 1000 ps/nm by 500ps/nm graduations. In this whole wavelength band, the compensation isonce shifted in the negative direction of the y axis, followed by fineadjustment in the positive direction of the y axis. The inclination ofthe dispersion characteristics of the dispersion compensating fiber hereis so selected as to cancel the inclination of the dispersionattributable to the transmission line.

The method of dispersion compensation illustrated in FIGS. 6 by which adispersion compensation having a large absolute value is first givenfollowed by dispersion compensations of a smaller absolute value each,reverse in plus/minus to the first is not limited to what is shown inFIG. 5 using circulators and mirrors. In the simplest way, twodispersion compensating fibers differing in plus/minus sign may bespliced.

In this embodiment, adjustment is done by about 500 ps/nm graduations,the graduation can be so selected as to achieve a target level oftransmission quality, for instance not more than 10⁻¹² in bit errorrate.

The dispersion compensation device and its hardware will be describedwith reference to FIG. 7. The same parts as in the configuration of FIG.5 will be denoted by respectively the same reference signs, and theirdescription will be dispensed with.

Referring to FIG. 7, optical signals inputted to the port 111, aftergoing through dispersion compensation, are outputted fromtheport113.Whileeachdispersioncompensationunitcontains a mirror in theconfiguration of FIG. 5, this embodiment has mirror ports 145 disposedon a case 115, and the mirror 140 is inserted into each mirror port 145from outside the case. When the mirror 140 is not inserted into themirror port 145, the latter is covered with a dustproof lid. This makesit possible to know at a glance which of the last dispersioncompensation units optical signals have reached. Incidentally, only thedispersion compensation unit of the final stage, having no switch, hasthe mirror 140 in the case 115. However, the dispersion compensationunit of the final stage may also have a mirror port on the case.

In the configuration shown in FIG. 7, a variable attenuator 150 isdisposed between the optical circulator 110 and the dispersioncompensation unit of the first stage. This is intended not only to varythe dispersion compensation quantity but also to keep the quantity ofsignal loss, which varies at the same time, constant in the position ofthe port 113. More specifically, the attenuation quantity of thevariable attenuator 150 is so adjusted as to cancel variations of thetotal fiber length shown in FIG. 6.

The variable attenuator 150 and the plural 1 ×2 optical switches 130 arecontrolled by a control unit 160. The dispersion quantity and theattenuation quantity of each dispersion compensation unit are recordedin a nonvolatile memory 180, and a central processing unit 170 computesthe total dispersion quantity by referencing the nonvolatile memory 180and instructs the control unit 160 to change over optical switches andthe variable attenuator 150 to set an attenuation quantity. The state ofdispersion quantity adjustment is displayed on a stated is play unit175. A communication control unit 190 delivers to the central processingunit 170 instructions from a superior monitoring and control unit forthe device.

The plural 1 ×2 optical switches 130 select a normal contact (outputselected when no electric power is supplied) for the dispersioncompensating module of the next stage. This makes it possible to supplypower only to the 1 ×2 optical switch which causes optical signals to beturned back by the mirror 140 out of the plural 1 ×2 optical switches130. Therefore, the optical components towhichpower is to be suppliedare limited to thevariable attenuator 150 and a maximum of one 1 ×2optical switch, enabling power consumption to be saved.

The packaging of the variable dispersion compensation device will now bedescribed with reference to FIG. 8. In order to be accommodable by astandard 19 ×25.4 mm rack, the variable dispersion compensation device100 includes twelve 5.4-mm rack dispersion compensation units 200 of 25mm (w) ×85 mm (h), one each of variable attenuator module l5O′, opticalcirculator unit 110′ and power supply unit 220 all of the same externalshape as the units 200, and a control unit 210 accommodating a controlunit, a central processing unit and so forth. In this variabledispersion compensation device, the mirror 140 is inserted into thefifth dispersion compensation unit. In this statethe optical switch inthe dispersion compensation unit is changed over to the mirror 140 side.The electrical connector and the optical connector of each unit areconnected to receptacles disposed on the back wiring board on the rearside of the device. Therefore, the sixth and subsequent dispersioncompensation units 200 can be inserted or withdrawn at any time withoutaffecting main signals. For this reason, when the routing of thetransmission line is to altered, the change to an appropriate dispersioncompensation unit can be done in advance, which facilitates remotecontrol.

Incidentally in FIG. 5 and FIG. 7, no 1 ×2 optical switch is illustratedfor the dispersion compensation unit of the final stage, but a mirror isprovided. However, to make the interface the same as others, the mirrorof the final stage not shown in mounted on the back wiring board side.

These embodiments of the invention can provide variable dispersioncompensation devices to compensate for chromatic dispersion ofwavelength division-multiplexed optical signals attributable to a DSFtransmission line.

Mirrors are provided on the front face of the case to make it possibleto know at a glance which, out of the plural dispersion compensationunits, was used last. There is a further advantage that any dispersioncompensation unit not in use, including the optical switch, can bewithdrawn, or an additional one provided, as desired.

The variable attenuator provides a benefit of restraining the loss to acertain level when the dispersion compensation quantity is to bealtered.

Since the dispersion compensating fibers are passed two ways, the lengthof dispersion compensating fibers can be reduced to half of their numberused in the conventional way.

1. A dispersion compensation device, wherein: one end of a first fiberhaving a positive chromatic dispersion characteristic and one end of asecond fiber having a negative chromatic dispersion characteristic areconnected to each other, first optical signals are inputted from the endof said first fiber, second optical signals are outputted from the endof said second fiber, and chromatic dispersion of said first opticalsignals is thereby compensated for.
 2. A dispersion compensation device,wherein: one end of a first fiber having a positive chromatic dispersioncharacteristic and one end of a second fiber having a negative chromaticdispersion characteristic are connected to each other, first opticalsignals are inputted from the end of said second fiber, second opticalsignals are outputted from athe end of said first fiber, and chromaticdispersion of said first optical signals is thereby compensated for. 3.A dispersion compensation device, wherein: optical signals inputted to afirst port of an optical circulator are outputted from a second port ofsaid optical circulator, made to pass a first fiber whose one end isconnected to the second port and a second fiber whose one end isconnected to an the end of said first fiber, turned back by a mirrorconnected to the other end of said second fiber, made to pass saidsecond fiber and the first fiber again, and outputted from a third portof thesaid optical circulator, wherein: a first chromatic dispersioncharacteristic of first fiber and a second chromatic dispersioncharacteristic of second fiber differ in positive/negative sign.
 4. Thedispersion compensation device according to claim 3, wherein: anabsolute value of a dispersion quantity of optical signals on thesaidfirst fiber is greater than an absolute value of a dispersion quanfty ofoptical signals on said second fiber.
 5. A dispersion compensationdevice, comprising: dispersion compensation units each further having adispersion compensating fiber, a 1 ×2 optical switch connected an inputport to one end of the dispersion compensating fiber, and a mirror portconnected to a first output terminal of the 1 ×2 optical switch; amirror; and an optical circulator, wherein: said optical circulatoroutputs optical signals inputted to a first port to a second port andoutputs optical signals inputted to the second port to a third port,said dispersion compensation units are connected in series to the secondport with another end of the dispersion compensating fiber as input andthe second output terminal of the 1 ×2 optical switch as output, and aplurality of dispersion quantities can be set by setting a plurality ofthe 1 ×2 optical switches and fitting said mirrors to one mirror port.6. A dispersion compensation method, wherein: optical signals aresubjected to positive dispersion compensation of which a firstdispersion compensation quantity is positive, and said optical signalssubjected to the positive dispersion compensation are subjected todispersion compensation of which a second dispersion compensationquantity, smaller in absolute value than the first dispersioncompensation quantity, is negative.
 7. A dispersion compensation method,wherein: optical signals are subjected to negative dispersioncompensation of which a first dispersion compensation quantity isnegative, and said optical signals subjected to the negative dispersioncompensation are subjected to dispersion compensation of which a seconddispersion compensation quantity, smaller in absolute value than thefirst dispersion compensation quantity, is positive.